1. Field of the Invention
The invention relates to flow meters and, more particularly relates to apparatus for monitoring the volume of liquids such as potable water that flow through devices such as filter assemblies and to a method of monitoring such volumetric flow.
2. Discussion of the Related Art
Reliable liquid flow measurement is important in many applications including the beverage handling industry, the pharmaceutical industry, the photo-processing industry, and many different liquid filtration industries. One example is the domestic or household potable water filtration industry in which replaceable filter cartridges are used to treat or purify water prior to its use. Typical filter cartridges have a rated useful life in terms of their volumetric capacity. That is, the filtration effectiveness of the carbon block or other filtration media of such cartridges decreases as the aggregate volume of treated water increases. Typical filter cartridges have a rated life of 500 gallons to 2000 gallons. Failure to replace filter cartridges at the end of their rated lives may result in reduction of treated water quality. Knowing or even estimating the time at which that a filter cartridge's volumetric capacity is reached can be difficult in most households because several different people use different amounts of water without informing one another of the volumes of water used by each.
The need therefore exists to monitor volumetric liquid flow through systems or possibly other parameters relating to liquid flow such as flow rate, and this need is especially evident with respect to the flow of water through a domestic or household type potable water filtration system. However, commercially-acceptably liquid flow meters heretofore have been unavailable for several reasons. First, flow meters tended to be relatively expensive. Second, they have tended to be relatively bulky and difficult to incorporate into existing filtration system designs. Third, and most importantly from the standpoint of regulatory agencies and others who demand accurate flow measurement through relatively wide ranges of source pressures and liquid flow rates, they must be accurate. However, accurate volumetric flow measurement over relatively wide ranges of source flow rates is a difficult task, particularly at medium to low flow rates of 1 gallon per minute or less and is especially difficult at low flow rates of 0.4 gallons per minute or less.
One attempt to design a flow monitor that meets at least some of the criteria described above is disclosed in U.S. Pat. No. 5,540,107 to Silverman et al. (the Silverman patent). The flow meter or flow monitor disclosed in the Silverman patent monitors rotation of a paddle wheel. Specifically, it counts paddle wheel revolutions, then determines aggregate volumetric liquid flow based upon precalibrated data representative of volumetric flow per pulse. This data is used in conjunction with a pre-stored rated volumetric filter cartridge capacity to provide an indication that the filter cartridge associated with the flow monitor requires replacement when the cartridge's volumetric capacity is reached.
The flow monitor of the Silverman patent attempts to minimize pressure drop through an acceptable range of operating flow rates. It includes a housing presenting an impeller chamber in which is disposed an impeller or paddle wheel that essentially acts as a paddle wheel. That is, water enters the impeller chamber peripherally at a first portion, engages the fins or vanes of the paddle wheel to drive the paddle wheel to rotate as the water flows through the impeller chamber, and then exists the impeller chamber radially or at least peripherally at another location. A magnet is inserted into the paddle wheel, and a sensor having an induction coil and a flux concentrator is disposed adjacent the paddle wheel so that the sensor counts two pulses with each rotation of the paddle wheel. The counted pulses can then be used to determine volumetric liquid flow.
The flow meter disclosed in the Silverman patent exhibits several drawbacks and disadvantages.
For instance, both its paddle wheel design and its associated detector require significant torque to move or drive the paddle wheel. Significant torque is required to drive the paddle wheel because the paddle wheel presents significant resistance to the generally peripheral flow of water therethrough. Additional resistance to liquid flow through the paddle wheel occurs because the paddle wheel floats in the paddle wheel chamber and is only loosely or roughly supported. The discrete magnets and induction coil-type detector also provide significant resistance to paddle wheel movement. If left uncompensated for, the pressure drops resulting from these resistances would reduce the liquid flow rate through the flow meter by an unacceptable magnitude.
The Silverman patent was cognizant of the need to minimize pressure drops and, hence, designed features into its flow meter to reduce back pressure. Most notably, it provides a tapered inlet nozzle which directs liquid into the impeller chamber at a direction tangent to the fins or blades of the paddle wheel to increase liquid flow rates. However, by converting the potential energy of the flowing liquid to kinetic energy by accelerating the liquid prior to its entry into the paddle wheel, Silverman's nozzle necessarily increases pressure drop in its flow meter. Moreover, in order to permit the tapered nozzle to operate effectively, an inlet plenum or reservoir must be formed in the housing upstream of the nozzle. Provision of the nozzle and the plenum necessarily complicates the flow meter design and proves only partially effective in reducing pressure drop. In addition, the overall design has a somewhat limited range of linearity (i.e., a flow rate range in which a uniform number of pulses are counted for each gallon of liquid flowing through the meter).
The Silverman patent also recognizes that problems are associated with the use of a discrete magnet and states that it would be preferable to have the paddle wheel as a whole magnetic. However, Silverman considered regulatory constraints on materials, an insurmountable obstacle to this task. (It is believed that the regulatory constraints referenced in the Silverman patent are those that would prohibit the use of a paddle wheel in which materials from the magnet leach or are washed into the water.) The Silverman patent therefore requires that the magnet be a separate insert imbedded in the paddle wheel.
In addition, the complexity of the flow meter disclosed in the Silverman patent renders it unduly bulky for many applications. It simply cannot be worked into many existing filter assembly designs without significantly modifying the assembly's design.
Moreover, the configuration of the paddle wheel magnet and the induction coil-type detector impose power restraints on the system that require the use of an external AC power source and an AC to DC converter if the flow meter is to be used to monitor liquids flowing at more than 3 gallons per minute. These power restraints limit the practical applicability of the flow meter disclosed in the Silverman patent to applications in which liquids flowing at less than 3 gallons per minute are monitored.